[mlir][Vector] Update VectorEmulateNarrowType.cpp (2/N) (#123527)

This is PR 2 in a series of N patches aimed at improving
"VectorEmulateNarrowType.cpp". This is mainly minor refactoring, no
major functional changes are made/added.

**CHANGE 1** 

Renames the variable "scale". Note, "scale" could mean either:

  * "container-elements-per-emulated-type", or
  * "emulated-elements-per-container-type".

While from the context it is clear that it's always the former (original
type is always a sub-byte type and the emulated type is usually `i8`),
this PR reduces the cognitive load by making this clear.

**CHANGE 2** 

Replaces `isUnalignedEmulation` with `isFullyAligned`

Note, `isUnalignedEmulation` is always computed following a
"per-element-alignment" condition:
```cpp
// Check per-element alignment.
if (containerBits % emulatedBits != 0) {
  return rewriter.notifyMatchFailure(
    op, "impossible to pack emulated elements into container elements "
    "(bit-wise misalignment)");
}

// (...)

bool isUnalignedEmulation = origElements % emulatedPerContainerElem != 0;
```

Given that `isUnalignedEmulation` captures only one of two conditions
required for "full alignment", it should be re-named as
`isPartiallyUnalignedEmulation`. Instead, I've flipped the condition and
renamed it as `isFullyAligned`:

```cpp
bool isFullyAligned = origElements % emulatedPerContainerElem == 0;
```

**CHANGE 3**
  * Unifies various comments throughout the file (for consistency).
* Adds new comments throughout the file and adds TODOs where high-level
    comments are missing.
    
    
**GitHub issue to track this work**:
#123630

Author: banach-space